1. Field of the Invention
This invention relates to an iron-type golf club and, more particularly, to an oversized iron-type golf club.
2. Description of Background and Related Art
There are a multitude of golf club designs, utilizing a wide variety of shafts and golf club heads. U.S. Pat. No. 5,039,162 to Fenton discloses a composite golf club shaft having a standard butt diameter, and an oversized tip diameter. By convention, and for purposes of this disclosure, tip diameter is the outer diameter of the shaft measured at the location just outside of the hosel opening. A standard shaft has a tip diameter of approximately 0.37 inches. Fenton teaches combining standard modulus of elasticity carbon fiber material with at least one ply of fiberglass to increase the diameter of the shaft tip. The resultant shaft tip has a diameter between 0.400 and 0.440 inches. The glass fiber is indicated to be desirable because it has a lower cost than carbon fiber. Undesirably, however, the weight of the shaft is increased significantly without a substantial increasing of the stiffness of the shaft.
Another prior art shaft design is disclosed in U.S. Pat. No. 5,265,872 to Tennent. Tennent discloses a composite golf club shaft having a base rod and a flared tip portion. Tennent teaches that the flared tip portion in the club will reduce breakage at the hosel region. On the other hand, the shaft design of Tennent is undesirable in that it is difficult to manufacture compared to a shaft having a conventional profile. The Tennent shaft is further undesirable in that it is difficult to manufacture shafts of this configuration with a consistent stiffness distribution in that the particular progression of the shape and the shaft can effect shaft deformation and consistency. The Tennent shaft is also further undesirable for a flared butt shaft construction. Flared butt shafts provide the advantage of lightening the upper potion of the shaft while achieving the same stiffness as a conventional shaft. Tennent's combination of a flared tip and a flared butt, however, would require the use of a separate mold for each shaft length as the ends of the shaft could not be cut without affecting the shafts' consistency within the set.
For some time, golf club manufacturers have combined oversized metalwood heads with composite shafts. These clubs are easier to hit than the conventionally designed woods that they replaced. The reason is two-fold. First, by enlarging the head, the sweet spot of the club face is enlarged. Second, the composite shaft is generally lighter than a conventional steel shaft, and therefore, can be swung more quickly with the same amount of force generating greater club head speed and, therefore, greater ball velocity.
Since the combination of oversized metalwood heads with composite shafts is so beneficial, many club manufacturers have combined oversized ironheads with composite shafts. As with the case for oversized woods, the desired effect is an iron that is less sensitive to off-center hits and generates greater ball velocity.
Applicant has determined, however, that the increase in the iron-type head volume is typically accompanied by displacement of the center gravity of the club head. Specifically, when the volume of the club head increases, the center of the gravity tends to shift away from the shaft axis. This occurs even when the mass of the club head remains the same as that of a standard iron head. Applicant has further determined that the shifting of the center of gravity further from the shaft axis causes the ironhead to droop when swung in a conventional manner. This droop effect is created by the centrifugal force acting through the center of gravity of club head during the swing. This force creates a bending moment on the tip of the shaft, which causes the club head to droop downward, so that the toe of the club face will be lower than the heel at time of impact with the ball. This can lead to one of two results. One, the misaligned club head can strike the ground prior to hitting the ball. Alternatively, the leading edge of the club, rather than the impact face, can strike the ball. Neither result will be appreciated by the golfer.
FIG. 1 illustrates the change in the position of the center of gravity (from 1 to 2) due to the use of an oversize head. The desired position of the iron with respect to the ball is shown in phantom. The oversize iron incorrectly positioned because of the droop effect is shown in solid lines. Droop effect occurs in standard irons but not to the extent that it occurs in oversized iron heads. This is because the shift of the center of gravity magnifies the droop effect in an oversized head.
Accordingly, one aspect of Applicant's invention is an improved golf club, particularly adapted for use with an oversized iron-type head including an iron-type head and a shaft particularly adapted to minimize the droop effect. The iron-type head defines a sole portion, an impact face, a back portion and a hosel extension defining an opening. The shaft has a butt portion and a tip portion. The tip portion has a substantially straight profile and defines an outer diameter d greater than or equal to 0.38 inches and less than or equal to 0.40 inches (i.e., 0.38 inches .ltoreq.d.ltoreq.0.40 inches). The shaft has a weight W less than or equal to 85 grams (W.ltoreq.85 grams). At least a part of the tip portion is positioned within the opening of the head. Desirably, the tip portion has a length of greater than or equal to 2 inches and less than or equal to 11 inches. Desirably, the tip portion defines a substantially straight and continuous profile of the fiber layers with no area of abrupt variation of diameter to prevent risks of weakened area or breakage area.
Another aspect of the invention is a golf club including an oversized iron-type head and a shaft. The head defines a sole portion, an impact face, a back portion and a hosel extension defining an opening. The impact face has an area of greater than or equal to 3200 mm.sup.2. A shaft has a butt portion and a tip portion. The tip portion has a substantially straight profile and defines an outer diameter greater than or equal to 0.38 inches and less than or equal to 0.40 inches. The shaft desirably comprises a single type of fibrous material and at least a part of the tip portion is positioned within the opening of the head. Advantageously, the fibrous material comprises carbon fibers, preferably carbon fibers having a standard modulus of elasticity. In its preferred embodiment, the golf club provides a light weight shaft having increased resistance to droop effect and greater torsional strength to avoid twisting of the golf club head at impact. Desirably, the shaft has a substantially straight tip portion comprised of a single composite material so that the stress distribution of the club is easily controlled and reproducible. Further, this permits various irons to be manufactured from a standard shaft which is then cut to the desired length before assembly.
An attempt to combine an oversized wood-type head whose volume is 220 cc or greater with a shaft having an enlarged tip between 0.37 to 40 inches was disclosed in the U.S. Pat. No. 5,573,467 to Chou. This combination is allegedly made for the purpose of improving the performance of the club; and in particular for increasing distance. But, this disclosure is substantially different from the present invention in the fact that Chou is limited to a wood-type club. The recognized standard for tip diameter for a wood has always been considered to be different; and more precisely less, for a wood than for an iron. For wood clubs, the tip standard is 0.335 inches while for the iron club the tip standard is 0.370 inches. Therefore, it is not intuitive to transpose the teaching of the Chou patent directly to an iron-type golf club since it has been recognized by the art that woods and irons should be manufactured according to different design criteria.
In addition, the Applicant has determined that an iron-type club performs substantially different than a wood-type golf club. In particular, an iron requires more stability when the ball impacts the club near the toe than a wood.
As illustrated in FIGS. 2 and 3, when a wood strikes a ball at the toe, the ball is still driven a relatively large distance and in the direction of the targeted trajectory. This is due to the fact that the center of gravity of a wood is more distant from the impact face than the center of gravity of an iron. This imparts a side spin (ss) on the ball which can be compensated for by properly designed bulge radius (R), but has a less significant result in the loss of ball velocity (v.sub.w).
In contrast, as shown in FIG. 3, when an iron strikes the ball at a location offset from the center of gravity toward the toe, the face is twisted backward by the impact a distance M relative to the center of gravity of the head, resulting in a more significant loss of ball velocity (v.sub.i). As the iron face becomes larger, the effect can be magnified due to the distance (h) between the potential impact location on the toe and the center of gravity, resulting in a larger moment arm.
In addition to the significant loss of velocity for off-center hits, the iron also has a substantial loss of accuracy due to the deviation angle .theta. of the ball as it rebounds off the rotated face. Even a very slight rotation of the club during the time the ball is in contact with the impact face, can result in a significant divergence of the trajectory T of the ball from the targeted trajectory T.sub.o.
Therefore, an iron-type club requires a shaft which is specifically adapted to prevent the head from rotating during an offset impact with the ball. The Chou patent does not address this issue (even for wood-type clubs). Therefore, there is no particular reason to believe that the teaching of Chou would be useful in solving this problem for irons.